The Bethe Ansatz
Bogoliubov theoryc.l.b
It is possible to effectively resum terms in perturbation theory by following Bogoliubov's logic for quasi-degenerate gases. This begins by assuming that the lowest momentum mode is macroscopically occupied, namely that
Keeping only leading terms and performing a Bogoliubov transformation then gives the simplified Hamiltonian
where
and where the ground state energy is
in terms of the effective interaction parameter
Derivation
Following Bogoliubov's logic, we assume that the lowest momentum mode is macroscopically occupied, namely that
In the Lieb-Liniger Hamiltonian represented in Fourier space l.hf, keeping only the leading terms involving at least two entries of the zero-momentum fields gives
Simple algebra then gives
where
This can be diagonalized by a Bogoliubov transformation
The diagonalized quadratic form in the Hamiltonian then becomes
The Hamiltonian itself simplifies to
where the ground state energy is
in terms of the effective interaction parameter
The Bogoliubov approach should provide an accurate approximation of the Lieb-Liniger model in the limit of small interactions. Note the the ground state energy has non-algebraic corrections in terms of the interaction parameter, reflecting the fact that naive perturbation theory around the noninteracting point fails. Bogoliubov theory cannot be accurate for large interactions, in any case certainly not for

Created: 2024-01-18 Thu 14:24